#include <bits/stdc++.h>
using namespace std;
 
int frogs(int N, vector<int> arr)
{
    if (N <= 1) return 0;
    if (arr[0] == 0) return -1;
 
    int maxReach = arr[0];
    int step = arr[0];
    int jump = 1;
    int i = 1;
    for (i = 1; i < N+1; i++) {
        if (i == N) return jump;
        maxReach = max(maxReach, i + arr[i]);
        step--;
        if (step == 0) {
            jump++;
            if (i >= maxReach) return -1;
            step = maxReach - i;
        }
    }
 
    return -1;
}
int main() {
	int n;
	cin>>n;
	vector<int> arr(n);
	for(int i=0;i<n;i++)
		cin>>arr[i];
 
	int result = frogs(n,arr);	
	cout<<result;
	return 0;
}

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